Benthic Fluxes of Oxygen, Ammonium and Nitrate and Coupled-uncoupled Denitrification Rates within Communities of Three Different Primary Producer Growth Forms

Inorganic nitrogen and oxygen fluxes together with coupled-uncoupled denitrification were studied in sediments covered by different primary producers (benthic microalgae, the floating macroalga Ulva rigida, and the rooted phanerogam Ruppia cirrhosa). High DIN (Dissolved Inorganic Nitrogen) assimilation rates were measured for all the primary producers and resulted in low denitrification rates, in particular at the Ulva and Ruppia colonised sites. The competition for NH 4 + and NO3 − between phototrophic organisms and nitrifiers-denitrifiers was particularly strong when DIN concentrations in the water column were low. Despite algal uptake denitrification rates were appreciable (>200 µmol N m−2h−1 in the site covered by benthic diatoms due to high availability of NO3 − in the water column and efficient coupling between nitrification and denitrification. In the sites with macrophytes losses of N due to coupled-uncoupled denitrification were negligible compared to assimilation rates. Most of the organic nitrogen pool in the Ulva biomass is probably recycled in the water column while a consistent part of the N stored in Ruppia may be buried in the sediment